let a=[...a_], b=[...b_];

function A(a,b) {
  return a.concat(b);
}

A(a,b);

let a=[...a_], b=[...b_];

function B(a,b) {
  return [...a, ...b];
}

B(a,b);

let a=[...a_], b=[...b_];

function C(a,b) {
  return (a.push(...b), a);
}

C(a,b);

let a=[...a_], b=[...b_];

function D(a,b) {
    Array.prototype.push.apply(a, b);
    return a;
}

D(a,b);

let a=[...a_], b=[...b_];

function E(a,b) {
    return b.reduce((pre, cur) => [...pre, cur], a);
}

E(a,b);

let a=[...a_], b=[...b_];

function F(a,b) {
    while(b.length) a.push(b.shift());
    return a;
}

F(a,b);

let a=[...a_], b=[...b_];

function G(a,b) {
    while(a.length) b.unshift(a.pop());
    return b;
}

G(a,b);

let a=[...a_], b=[...b_];

function H(a, b) {
    var len = b.length;
    var start = a.length;
    a.length = start + len;
    for (var i = 0; i < len; i++ , start++) {
        a[start] = b[i];
    }
    return a;
}

H(a,b);

let a=[...a_], b=[...b_];

function I(a, b){
   var oneLen = a.length, twoLen = b.length;
   var newArr = [], newLen = newArr.length = oneLen + twoLen;
   for (var i=0, tmp=a[0]; i !== oneLen; ++i) {
        tmp = a[i];
        if (tmp !== undefined || a.hasOwnProperty(i)) newArr[i] = tmp;
    }
    for (var two=0; i !== newLen; ++i, ++two) {
        tmp = b[two];
        if (tmp !== undefined || b.hasOwnProperty(two)) newArr[i] = tmp;
    }
    return newArr;
}

I(a,b);